401(k) Investing

I confess, I know almost nothing about investing.

At some point, I really need to start actively managing my 401(k). I just dumped 5% of my salary in it, and signed up for a 100% agressive portfolio - because as someone in their mid-20s who can handle the fluctuations, that's what I'm supposed to do.

I've done some reading online today, though, and here's one thing I realized: at least up to the 5% I'm currently contributing, my employer matches dollar for dollar. And that seems rare - most other numbers I'm seeing are closer to 50 cents on the dollar.

So, I've decided I need to take better advantage of this. First, I need to figure out exactly how much I can contribute, and up to what percent my employer will match. Then, max that out.

I'd like to find a calculator online that will help me figure out the real impact of increasing my contribution. If I contribute $100 more into my 401(k), I know that my actual take-home pay will not decrease by $100, since 401(k) contributions are pre-tax. But what's the ratio? Is it $65 for that $100? I need to figure this out before I change my contributions.